Digital subscriber line (xDSL) is a high-speed data transmission technology for transmitting data on a telephone twisted pair. Except for the baseband DSL, such as ISDN digital subscriber line (IDSL, at a transmission rate of 144 Kbps) and single-line high bit rate digital subscriber line (SHDSL), the passband xDSL uses the frequency division multiplexing technology to allow e the xDSL to coexist with the plain old telephone service (POTS) on a twisted pair. The xDSL occupies the high band, and the POTS occupies the baseband part below 4 KHz. The POTS signals are separated from the xDSL signals through a splitter. The passband xDSL adopts discrete multi-tone (DMT) modulation. A DSL access multiplexer (DSLAM) system provides multiple channels of xDSL access. A reference model of the DSLAM system is shown in FIG. 1.
In FIG. 1, NMS represents a network management system, xTU-C represents a transmission unit of the xDSL at an access point, xTU-R represents a transmission unit of the xDSL at the far end, and Splitter is a filter that separates high-frequency xDSL signals from low-frequency POTS signals.
When the xDSL is activated, a certain rate is determined, for example, a downlink rate of 2 Mbps. When an xDSL transceiver is operating in a training mode, a line rate is determined according to the line conditions, for example, 4 Mbps. Once determined, the line rate is constant in the actual operation of the xDSL, regardless of the service data rate. Technically speaking, to keep a constant line rate, the transmit power on the line must keep unchanged.
The data traffic on the line varies greatly, depending on the time period in a day and the type of the service on the network. In most cases, the average data rate is lower than the line rate. When the line is idle, the data rate is zero. Therefore, if the transmit power on the line remains unchanged in any case, a waste of power occurs when the line data rate is low or zero. To reduce energy consumption, the prior DSL standards provide three operating modes: L0 mode (full power mode), L2 mode (low data rate state), and L3 (sleep state).
L0 is a normal operating mode. The L0 keeps the line rate unchanged and provides the adaptation functions such as seamless rate adaptation (SRA) and bit swap but does not provide energy efficiency. L2 is a low power consumption mode and provides no adaptation functions. L3 is an idle mode and cannot provide data services. When the data transmission rate is moderate, the xTU-C or the xTU-R may request to enter the L2 mode to reduce the transmit power of the power supply and save energy at the cost of reducing the transmission rate. After the line enters the L3 mode, because L3 is an idle mode which provides no data transmission service and does not need to send signals, the energy efficiency is the highest but no data transmission is available. For example, when the line is in the full rate state (for example, the user is downloading a large video file), to ensure fast and accurate data transmission, the ADSL2 system works in the L0 full power mode. When the line rate is low (for example, the user is reading a document online), the transceiver power is adjusted to the L2 low energy consumption mode automatically. When the user logs out, the system shifts to the sleep state quickly and the transceiver power is adjusted to the L3 low energy consumption mode. In other words, according to the actual data traffic on the line, the transmit power may be selectively switched between L0, L2 and L3 Switching operations are finished within 3 seconds to avoid affecting the services.
The prior xDSL standards provide the three modes primarily to save energy, but the modes are defective in some aspects. First, the modes are categorized roughly and cannot meet the actual requirements, and therefore the effect of energy saving is not apparent. Second, the L2 mode saves energy at the cost of reducing the transmission rate. Therefore, the practicality of utilizing the L2 mode is minimized and not applicable to most services due to transmission rate requirements. Moreover, the switching between the modes is rather complicated and time-consuming.